well, energy efficient homes may have a dangerous side effect. they could be making the folks who live there sick. alicia acuna from denver with more. alicia? >> hi, megyn, who knew that our attempt at more efficient living could make things harder on our bodies, what doctors at national jewish health are finding. in the 1970's, builders answered the call for greener homes, that meant better insulation, but it also meant less airflow. listen. >> because of that, we probably have created a problem indoors because we're going to have more air pollution because it's tighter. we don't have the ventilation and a way to get rid of the indoor air pollution so that it goes outdoors. >> reporter: what stays inside are things like pet dander, dust, mold and bacteria, increasing problems for allergy sufferers and 26 million americans suffering from asthma. in a study with several children who carried air monitors, the doctor found that the air pollution indoors is much worse than outdoors. this mother found out her son is allergic to many things indoors. >>